The Death Wobbles - How to regain control?

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Re: The Death Wobbles - How to regain control?

Postby Byke » Thu Nov 22, 2012 9:57 pm

Byke wrote:It's just that the particular speed that this will occur at will vary for every combination of:
- bike,
- rider, and
- position

The theory went that should you develop a speed wobble, which is a form of simple harmonic motion, you merely need to alter any one of the three factors.


As a couple of others have pointed out changing speed will also stop the wobbles. In my - thankfully very limited - experience, getting marginally out of the saddle is probably easier to do quickly than either speeding up or slowing down.
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by BNA » Fri Nov 23, 2012 5:21 am

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Re: The Death Wobbles - How to regain control?

Postby singlespeedscott » Fri Nov 23, 2012 5:21 am

ColinOldnCranky wrote:I gather that you don't get the resonance on the way up to 70kph.

The thing about this effect is that it doesn't occur ABOVE a certain speed. It occurs AT a certain speed. Go past that speed and it settles down again. But at higher speeds the amount of energy being pumped into the system is higher than at lower speeds. Once the energy/second put in is exceeding the energy/second being expended through dampening then the problem manifests itself.

Of course, it does not have to be the whole bike either - we would be in trouble if onlt the wheel did the same thing. Indeed if your front wheel in your car is not balanced properly...

It hasn't occurred at any speed I've ridden the frame too.
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Re: The Death Wobbles - How to regain control?

Postby william » Fri Nov 23, 2012 5:46 pm

rockdoctor said what I feel.

I bought my first road bike only 2 years ago and on the first downhill ride the bike felt like I had a rapidly deflating tyre at 50 kph. The bike was all over the road scaring the poo out of me. A sudden stop to fix the flat and found nothing wrong.
Long story short, ugly wheels branded "Mavic Aksium Race" wheels.
I got some lovely hand built wheels and no problems since.
Bikes, if designed properly, should not have any induced wobble of any sort. If it does, get it fixed or get a properly designed bike with better wheels.
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Re: The Death Wobbles - How to regain control?

Postby sturmey archer » Sat Nov 24, 2012 8:59 am

Isn't the major resonance likely to be due to the 60-100Kg of jelly (the rider) wobbling around on top of a relatively rigid frame? This lump of jelly is connected back to the steering mechanism by variably damped linkages (arms). Any slight wobble of the path of the frame cases the rider to move , but they arn't rigidly attached to the frame , there connected by arms and legs and bum , the muscle of which will form damped springs .
At this point dim memories of exercises in physics of multiple linked weights and springs start bubbling into my mind! Anyway the system is obviously quite complex so a change like lifting you bum will move the resonance of the whole system enough for the current wobble to be damped out. Of course it may almost immediately start again at another frequency , but hopefully ther rsonance at that frequency won't be as strong.
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Re: The Death Wobbles - How to regain control?

Postby ironhanglider » Sat Nov 24, 2012 9:27 am

william wrote:rockdoctor said what I feel.

I bought my first road bike only 2 years ago and on the first downhill ride the bike felt like I had a rapidly deflating tyre at 50 kph. The bike was all over the road scaring the poo out of me. A sudden stop to fix the flat and found nothing wrong.
Long story short, ugly wheels branded "Mavic Aksium Race" wheels.
I got some lovely hand built wheels and no problems since.
Bikes, if designed properly, should not have any induced wobble of any sort. If it does, get it fixed or get a properly designed bike with better wheels.


If it was simply a design issue it would have been solved long ago and no bikes would ever shimmy. If it was well understood and easily solved, the lawsuits for negligence would have made sure of that. The fact of the matter is that the rider is a large component in the system. A bike that shimmys under one rider will be rock solid under another, even if their proportions are identical.

I believe that every bike/rider combination will experience shimmy at some speed. For most riders/bikes the critical speed appears to be unattainable because they have not experienced it. The problem can't be 'solved' you just change the critical speed to something that you can't get to.

To change the critical speed requires even the smallest change. In your case it was wheels, for others the change can be made by, different tyres/tubes, different stems (presumably changing the weight distribution), sometimes even a different mental attitude. A friend of mine experienced shimmy briefly during a race, she had got away from me going uphill and was descending down the other side. She did not experience shimmy until I came past with a closing speed of 15km/h or more. What caused it? Was it my disturbing the air? My theory is that the surprise caused her to tense up a bit, changing the 'spring constant' of her arms. She went down the same hill again later in the race (at probably a similar speed) without experiencing shimmy.

Whatever solution works is a good one.

Cheers,

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